This invention relates to coating substrates by sputtering, and in particular to producing gradient coatings, that is, coatings of varying thickness across a surface of a substrate. An example of a field in which this capability is useful is in applying transparent, electroconductive coatings to transparencies to serve as resistance heating means, wherein it may be desired to provide greater resistance in some areas of the transparency than in others.
U.S. Pat. No. 4,744,880 (Gillery et al.) discloses a shaped anode in a sputter coating arrangement for producing a gradient coating. The gradation is effected by generalized shaping of the electric field, therefore that approach is not suitable for complex patterns or for sharply defined gradients. Also, for coating substrates that require scanning the sputtering electrodes over the substrate, a limited range of gradient patterns would be possible.
U.S. Pat. No. 4,600,490 (Gillery et al.) discloses an anode configured as a mesh in a sputtering electrode arrangement, and mentions that the mesh anode may be shaped in order to produce gradients in the deposited film. Because this arrangement is based on shaping the anode, it would have the same drawbacks of the patented arrangement discussed above.
The use of masks to produce patterned coating is known in the sputtering art as disclosed in U.S. Pat. No. 4,428,809 (Heimbach et al.). These masks are usually constructed of solid sheet material with openings cut out. This type of mask is not as versatile as would be desired for making gradients in the deposited coating thickness.
The effect of cathode shields on coating thickness is discussed in the "Handbook of Thin Film Technology," by Maissel et al. (McGraw-Hill, New York, 1970) on pages 4-13 to 4-15. It is proposed there that a shield with one or more apertures covering the face of a cathode would permit sputtering onto selected areas of a substrate. As in the case of shaped anodes, this approach is not as adaptable as would be desired for producing gradient coatings, particularly when the cathode and the substrate move relative to each other during sputtering.
U.S. Pat. No. 4,426,274 (Ephrath) shows a perforated anode with differential blocking of some of the apertures in a vacuum discharge system. However, the patent deals with etching rather than coating and the objective is to render the etching more uniform rather than to produce gradient effects.
It would be desirable to be able to apply sputtered coatings with greater flexibility than has heretofore been available as to the shapes of areas having different coating thicknesses, the amount of variation of coating thickness in different areas of a substrate, and the degree of gradation between areas of different thickness.